Alloys are known which offer good resistance to carburization by carburizing agents even to temperatures of the order of 1000.degree. C. Such alloys, however, do not possess all the characteristics required for certain applications. For example, such applications include the structural elements used in installations intended for very-high-temperature processing in oxidizing and/or carburizing mediums, for example the tube or pipe stills employed in petrochemical plants. Some of such characteristics are, on the one hand, creep strength within various temperature ranges including very-high temperatures, and on the other weldability. Furthermore, at least one alloy which has good resistance to carburization in atmospheres having an extremely low partial pressures of oxygen contains relatively high amounts of cobalt and molybdenum. Thus the alloy is at the same time relatively expensive, sensitive to vagaries in the supply of cobalt and possibly subject to catastrophic oxidation owing to the high molybdneum content.
More specifically, an important requirement of equipment such as pyrolysis tubes in petrochemical plants is that the alloy from which such equipment is made must form a scale under specific conditions of use which is resistant to spalling or degradation when the conditions of use are altered. For example pyrolysis tubes must be cleaned periodically to remove deposited carbon. The cleaning is most readily accomplished by increasing the oxygen partial pressure of the atmosphere within the tubes to effect one or more of the reactions EQU 2C+O.sub.2 .revreaction.2CO EQU 2CO+O.sub.2 .revreaction.2CO.sub.2 EQU C+O.sub.2 .revreaction.CO.sub.2
all of which result in changing a solid carbon deposit into a gas. Those skilled in the art will appreciate that the foregoing reactions are overly simplified in that "Carbon" deposits are almost never pure carbon but rather are complex, solid materials containing carbon and hydrogen and, usually, significant amounts of nitrogen, oxygen, phosphorous, and other elements present in the feedstock of the pyrolysis unit. Those skilled in the art will thus appreciate that the gas phase in a pyrolysis unit during burnout is a complex mixture of the product gases indicated in the forgoing equations and materials such as water vapor, nitrogen and nitrogenous gases and the like. A further factor which will be appreciated by those skilled in the art is that the latter two of the foregoing three equations are strongly exothermic when proceeding to the right. This exothermicity is further enhanced by the hydrogen content of the "carbon" deposits in pyrolysis tubes. Thus, although it is standard practice to control the oxygen partial pressure during carbon burnout of pyrolysis tubes in order to prevent runaway temperatures, variation in the character of the "carbon" deposit can result in the "hot spots", i.e., sites hotter than average, and "cold spots" i.e., sites cooler than average, during carbon burnout. Aside from considerations involved in the oxygen partial pressure during carbon burnout, there is a great range of oxygen partial pressures which can be expected in service in uses such as heat treating, coal conversion, steamhydrocarbon reforming and olefin production. For greatest practical use, an alloy should have carburization resistance not only in atmospheres where the partial pressure of oxygen favors chromia formation but also in atmospheres which are reducing to chromia and favor formation of Cr.sub.7 C.sub.3. In pyrolysis furnances, for example, where the process is a non-equilibrium one, at one moment the atmosphere might have a log of Po.sub.2 (atm) of -19 and at another moment the log of Po.sub.2 (atm) might be -23 or so. Such variable conditions, given that the Po.sub.2 (atm) of Cr.sub.7 C.sub.3 --Cr.sub.2 O.sub.3 crossover is about -20 at 1000.degree. C., require an alloy which is a universal carburization resistant alloy. Provision of such an alloy and objects made therefrom are the objects of the present invention.
A still further requirement for alloy to be of practical use is that the alloys be readily weldable by standard welding techniques, for example by gas tungsten arc (GTA), metal inert gas (MIG) and submerged are (SA) methods. Such weldability is essential. Unless equipment can be readily fabricated from standard shapes of alloys, all other alloy characteristics are of merely academic nature.